The invention relates to an optical scanning device for scanning an optically scannable information carrier, which scanning device is provided with a radiation source, a collimator lens unit with a first optical axis, an objective lens unit with a second optical axis, and an actuator for displacing the collimator lens unit, in a direction parallel to the first optical axis, from a first position to at least a second position, which actuator is provided with a first part and a second part, which are displaceably guided with respect to each other in a direction parallel to the first optical axis.
The invention also relates to an optical player comprising a table which can be rotated about an axis of rotation, an optical scanning device for scanning an optically scannable information carrier which can be placed on the table, and a displacement device by means of which at least an objective lens unit of the scanning unit can be displaced, in operation, with respect to the axis of rotation mainly in a radial direction.
An optical player and an optical scanning device employed therein of the types mentioned in the opening paragraphs are known from U.S. Pat. No. 5,754,513. An information carrier which can be scanned by means of the known optical scanning device comprises a transparent substrate on which an information layer with information tracks is provided. The scanning device scans the information layer from the side of the information carrier where the transparent substrate is situated, in which scanning operation, a radiation beam supplied by the radiation source passes through the transparent substrate. The known optical scanning device can suitably be used to scan information carriers of two different substrate thicknesses. For this purpose, the collimator lens unit of the known scanning device can be positioned in two positions, viewed parallel to the first optical axis. A displacement of the collimator lens unit causes a radiation beam""s convergence angle present between the objective lens unit and the substrate to be adapted to the substrate thickness such that the radiation beam is focused to a minimum scanning spot on the information layer of the information carrier. The actuator of the known scanning device, by means of which the collimator lens unit can be displaced, is not described in greater detail in U.S. Pat. No. 5,754,513.
It is an object of the invention to provide an optical scanning device and an optical player of the types mentioned in the opening paragraphs, which are provided with a functional, reliable and efficient actuator for displacing the collimator lens unit.
To achieve this object, an optical scanning device in accordance with the invention is characterized in that the first part of the actuator comprises a permanent magnet and a first magnetizable part of a magnetic circuit, while the second part of the actuator comprises an electric coil and a second magnetizable part of a magnetic circuit, both parts of the actuator mutually exerting a magnetic force in predetermined regions around the first position and around the second position, and in a non-energized state of the coil, which magnetic force causes both parts of the actuator to move with respect to each other into, respectively, the first and the second position.
To achieve this object, an optical player in accordance with the invention is characterized in that the optical scanning device used therein is an optical scanning device in accordance with the invention.
As said magnetic force in said regions causes the two parts of the actuator to be moved with respect to each other into, respectively, the first and the second position, the magnetic force provides stable mutual equilibrium positions of the two parts of the actuator in the first and the second position. As said equilibrium positions occur in a non-energized state of the coil, maintaining said equilibrium positions does not require electric current in the coil, i.e. no electric energy. A mutual displacement of the two parts of the actuator from the first position to the second position, or conversely, takes place by means of a comparatively short current through the coil. Interaction between said current and the magnetic field of the permanent magnet causes an electromagnetic driving force, under the influence of which both parts of the actuator are displaced with respect to each other, against said magnetic forces, from one equilibrium position to the other equilibrium position. In this manner, a functional, reliable and efficient operation of the actuator is obtained.
A particular embodiment of an optical scanning device in accordance with the invention is characterized in that the first magnetizable part of the magnetic circuit is U-shaped and provided with two legs which extend substantially perpendicularly to the first optical axis, and the permanent magnet is arranged between the two legs and magnetized in a direction which is substantially parallel to said legs, while the electric coil is secured to the second magnetizable part of the magnetic circuit and provided with wire portions extending substantially perpendicularly to the first optical axis and substantially perpendicularly to the legs. In this particular embodiment, the magnetic circuit includes a magnetic field with field lines which, in a transition region, make a U-turn from the end of both legs of the U-shaped part of the magnetic circuit, so as to be directed inwards, i.e. towards the permanent magnet. With respect to the U-shaped part, the second magnetizable part of the magnetic circuit is displaceably guided in said transition region in a direction parallel to the first optical axis, i.e. in a direction substantially perpendicular to the legs of the U-shaped part. At the location of each one of the two legs, a stable equilibrium position of the second magnetizable part with respect to the U-shaped part is obtained at the location where the concentration of the field lines in said transition region is highest. In this particular embodiment, the actuator is of a simple structure, and the equilibrium positions obtained are particularly stable.
A further embodiment of an optical scanning device in accordance with the invention is characterized in that the first part and the second part of the actuator are displaceably guided, with respect to each other, in a direction parallel to the first optical axis by means of two leaf springs extending transversely to the first optical axis. The leaf springs provide a particularly rigid mutual support of the two parts in directions perpendicular to the first optical axis, and they also provide the actuator with a particularly simple structure.
Yet another embodiment of an optical scanning device in accordance with the invention is characterized in that, in the first position and in the second position, the leaf springs exert an elastic pretension on both parts of the actuator, causing both parts of the actuator to move with respect to each other into, respectively, the second position and the first position. As the pretension causes both parts of the actuator in the first position and in the second position to move with respect to each other into, respectively, the second position and the first position, said pretension is helpful in mutually displacing the two parts from one equilibrium position to the other equilibrium position, so that the current through the coil necessary for such a displacement is limited.
A particular embodiment of an optical scanning device in accordance with the invention is characterized in that the scanning device is provided with a control unit, which is embodied so as to control an electric control current in the coil in such a way, that the two parts of the actuator proximate to the first position and proximate to the second position can be positioned, with respect to each other, in a position wherein a spherical aberration of a radiation beam supplied by the radiation source in a transparent substrate of the information carrier can be corrected in a predetermined way. In this particular embodiment, the actuator is not only used to adapt the optical scanning device to various substrate thicknesses of the information carriers by means of comparatively large displacements of the collimator lens unit, but also to correct spherical aberrations of the radiation beam in the transparent substrate of an information carrier by means of comparatively small displacements of the collimator lens unit around the first and the second position. In this embodiment, the actuator thus fulfills a dual function and hence is optimally used.